Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins, particularly ethylene and propylene. Promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including natural gas. Because of the relatively low-cost of these sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical source for light olefin production.
One way of producing olefins is by the conversion of methanol to olefins (MTO) catalyzed by a molecular sieve. Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO's). For example, U.S. Pat. No. 4,499,327 to Kaiser, fully incorporated herein by reference, discloses making olefins from methanol using a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1.
SAPO molecular sieves contain a three-dimensional microporous crystal framework structure of [SiO2], [AlO2] and [PO2] corner sharing tetrahedral units. The number of [SiO2] tetrahedral units is related to the acidic properties of the SAPO molecular sieve: the higher the Si content, the higher the molecular sieve acidity.
There are a wide variety of SAPO molecular sieves known in the art. Of these the more important examples as catalysts for the conversion of oxygenates to olefins include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. SAPO molecular sieves having the CHA framework type and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 Å, and cylindrical cages within the structure of approximately 10×6.7 Å type (“Atlas of Zeolite Framework Types”, 2001, 5th Edition, p. 96–97). SAPO-34 crystals have a cubic-like morphology and typically crystallize as cubes, partial cubes, platelets or flakes, depending on the height of the crystals. Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
Another important class of SAPO molecular sieves consists of mixed or intergrown phases of molecular sieves having the CHA and AEI framework types. Examples of such materials are disclosed in WO98/15496, published 16 Apr. 1998, and in PCT WO02/070407, published Sep. 12, 2002, both herein fully incorporated by reference.
U.S. Pat. Nos. 5,126,308 and 5,191,141 to Barger et al., herein fully incorporated by reference, disclose a method for converting methanol to light olefins using an ELAPO catalyst. The catalyst comprises a metal aluminophosphate molecular sieve having the empirical formula (ELxAlyPz)O2 where EL is a metal and x, y and z are mole fractions of EL, Al and P respectively. Preferred EL metals are silicon, magnesium and cobalt, with silicon especially preferred. According to these documents, small particle size SAPO-34 having low Si/Al ratios have a longer life and increased selectivity over other types of SAPOs when used as catalysts in MTO processes. These documents also describe a process for the manufacture of SAPO-34 of median particle diameters, expressed as a mass distribution, in the range of about 0.6 to 1.4 μm, in which the molecular sieve synthesis mixture is stirred. As the amount of metal is lowered, the particle size is also reduced.
Wilson, et al. reported that it is beneficial to use SAPO molecular sieves having low Si content for MTO conversions (Microporous and Mesoporous Materials, 29, 117–126, 1999, incorporated herein by reference). According to Wilson, SAPOs with low Si content deactivate slower and produce less undesired products than other SAPOs when used in MTO conversions.
PCT WO 01/23500 published Apr. 5, 2001 discloses a method for making an olefin product from an oxygenate-containing feedstock. In the method, a silicoaluminophosphate molecular sieve catalyst is contacted with the oxygenate-containing feedstock in a reactor at an average catalyst feedstock exposure (ACFE) index of at least 1.0. For a given catalyst used under such conditions, the method produces lower coke yield and provides an olefin product which is lower in C1–C4 paraffin content than when the ACFE index is lower than 1.0. According to example 1 and FIG. 1 of this document, SAPOs with low Si/Al atomic ratios are preferred in order to mimimize selectivity to propane.
We have now found that crystalline metalloaluminophosphate molecular sieves, preferably crystalline silicoaluminophosphate (SAPO) molecular sieves, having a small particle size and a high metal, preferably Si, content have excellent catalytic performances when used in MTO processes.